While traditional linear and large rotary motor structures are functional, they have several drawbacks in their implementation. Linear motors, which are also commonly referred to as linear actuators, generally fall into two categories: moving coil and moving magnet. Moving coil actuators (i.e., linear motors) generally produce less force per ampere-turn (“AT”) than the moving magnet category. Other drawbacks of conventional moving coil linear motors include poor thermal dissipation properties and relatively high bias forces, which are created by the current-carrying lead wires and cables. By contrast, moving magnet linear motors are generally free from such bias and can produce higher force per ampere-turn. But moving magnet linear motors have several drawbacks, too. For example, the positioning of the magnets relative to back-iron structures can contribute to some instability in motor operation, such as cogging. Another drawback is that the movements of the magnet structure can temporarily magnetize back-iron structures. Typically, a current controller is required to compensate for the unwanted effects of back-iron magnetization to more precisely manage the positioning of the actuator.
Rotary motors, which are another type of electric motor, include magnetic poles that are positioned at relatively large diameters about (or radial distances from) a rotor shaft. These magnetic poles, as well as the permanent magnets giving rise to those magnetic poles, are typically arranged coaxially about the shaft alternating in polarity and are positioned adjacent to magnetic field poles. An armature disk usually supports the permanent magnets as separate magnets in a plane perpendicular to the rotor shaft. Structures such as this are designed based on a certain tenet of electric motor design. According to this tenet, an increase in output torque is achieved by increasing the radial distance between the magnetic poles and the rotor shaft. Consequently, the magnetic poles of this type of electric motor are increasingly being positioned at larger distances from the rotor shaft to increase the torque arm distance from the axis of rotation to the air gaps, thereby increasing the output torque. A drawback to this approach is that additional materials are consumed in forming larger motor structures to accommodate the larger torque arm distance, such as those structures that are used to form magnetic flux return paths. These magnetic flux return paths are typically formed using “back-iron” to complete a larger flux path, which is generally circuitous in nature. By adding back-iron to complete a magnetic circuit, the magnetic material volume through which the magnetic flux passes increases, which detrimentally tends to increase the hysteresis and eddy current losses, both of which can be collectively referred to as “core losses.” Further, the addition of back-iron to complete a magnetic circuit increases the length of the magnetic flux path, thereby exacerbating core losses. Another drawback to motors of this type is that the motor volume increases as the magnetic poles are positioned farther from the shaft, which in turn, limits the available applications and uses for this type of motor.
“Back-iron” is a term commonly used to describe a physical structure (as well as the materials giving rise to that physical structure) that is often used to complete an otherwise open magnetic circuit. Back-iron structures are generally used only to transfer magnetic flux from one magnetic circuit element to another, such as either from one magnetically permeable field pole to another, or from a magnet pole of a permanent magnet to a magnet pole of another permanent magnet, or both. Further, “back-iron” structures are not generally formed to accept an associated ampere-turn generating element, such as one or more coils.
In view of the foregoing, it would be desirable to provide improved techniques and structures that minimize one or more of the drawbacks associated with conventional linear and rotary motors so as to, for example, increase either linear force output or torque output as well as to enhance efficiency either on a per unit size or per unit weight basis, or both.